Powder metallurgy techniques have been used to formulate metal composite materials with both high hardness and high impact strength. For example, U.S. Pat. Nos. 3,779,715, 4,024,902, and 4,140,170 describe composite articles containing metal carbide particles (e.g., tungsten carbide particles) surrounded by layers or matrices of softer metals. Such articles, though hard and impact resistant, are not readily machineable (as indicated by the disclosures of these patents). Also, the casting processes by which such articles are made are directed towards armorplate, large cutting tool bits, and the like, and are not well suited to the precision replication of parts such as die cavities.
U.S. Pat. No. 3,258,817 describes composite materials containing refractory particles surrounded by a binder metal. The refractory granules are said to be preferably interconnected in a continuous hard metal phase. Also, U.S. Pat. Nos. 3,823,002 and 3,929,476 describe precision shaped articles, such as electrical discharge machining electrodes, made by molding in a flexible mold a plastic mixture of multimodal refractory powders and a thermoplastic binder to form a green molded article of predetermined shape and dimensions, heating the green molded article to remove the binder and consolidate the refractory powders into an interconnected skeletal structure, and infiltrating the resulting skeletal structure with a molten infiltrant which is a low melting point metal or alloy. The articles of these three patents would not have high impact resistance, due to the interconnection or intergrowth of refractory granules. These articles would therefore be poorly suited for use as die cavities.
U.K. published Patent Application No. 2,005,728 A describes molded, non-refractory metal articles made by molding in a flexible mold a plastic mixture of non-refractory, spherical metal powders and heat-fugitive binder comprising thermoplastic material to form a green article of predetermined shape and dimensions, heating the green article to remove the binder and consolidate the non-refractory spherical powders in the form of a porous, monolithic skeleton of necked particles of non-refractory metal, infiltrating the skeleton with a molten metal having a melting point that is at least 25.degree. C. less than the melting point of the lowest melting of the spherical, non-refractory particles, and cooling the metal infiltrated skeleton thereby forming a homogeneous, void-free non-refractory metal article of two intermeshed metal matrices. The molded skeleton may be made of particles of Fe, Co, Ni, or their alloys and the infiltrant metal may be Cu, Ag, or Sn. Such articles are well suited for use as die cavities. Commercial embodiments of such articles prepared from "Stellite 1" brand cobalt particles and 80/20 copper/tin infiltrant have a Rockwell C hardness of about 40 and a Charpy unnotched impact resistance of about 4.75 joules (3.5 ft. lbs.).
A need exists for a process and articles which provide precision molded articles (such as die cavities) with Rockwell hardness and impact resistance above that attained by the articles of U.K. Published Patent Application No. 2,005,728 A, while preserving or enhancing the fidelity of replication attained through the method described therein. Also, it would be desirable to obtain precision molded articles with improved Rockwell hardness and impact resistance which can be machined using conventional tional cutting tools. In addition, it would be desirable to provide precision molded articles using materials which have not previously offered sufficient dimensional stability when employed in the process of said U.K. Published Patent Application No. 2,005,728 A.